Photorespiration and the Effect of Oxygen on Photosynthesis
Photorespiration is the light-dependent, oxygen-sensitive CO2 evolution from green leaves that originates from the metabolism of compounds in the glycolate pathway. It has been termed “an inevitable consequence of the existence of atmospheric oxygen” (1) and has been attributed to the ability of ribulose bisphosphate carboxylase to act also as an oxygenase catalyzing the reaction of oxygen with ribulose bisphosphate (RuBP) to yield phosphoglycolate (2), the precursor of the substrate for the glycolate pathway. In this reaction, oxygen not only produces the substrate for photorespiration but also competitively prevents the fixation of CO2 (3, 4). It has been known for some time that the inhibition of photosynthesis by oxygen is comprised of two components, inhibition of true photosynthesis and stimulation of photorespiration (5, 6). With the discovery of the oxygenase activity of RuBP carboxylase, both these components were attributed to the effect of oxygen on the enzyme (4, 7), and the joint action of oxygen and CO2 on the enzyme was proposed to be responsible for the regulation of soybean net photosynthesis (8). The oxygen concentration around a leaf—and in it, as the oxygen concentration is similar, (9)—can be quickly changed; and, if oxygen acts only on the oxygenase, one might expect a change in oxygen concentration to be accompanied by a corresponding rapid change in CO2 fixation to a new steady rate.
KeywordsOxygen Concentration Final Rate Photosynthesis Rate Stomatal Aperture Steady Rate
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